diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index abedf405cde..39abb68de70 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7345,12 +7345,8 @@ namespace ts {
             return type.id;
         }
 
-        function binarySearchTypes(types: Type[], type: Type): number {
-            return binarySearch(types, type, getTypeId, compareValues);
-        }
-
         function containsType(types: Type[], type: Type): boolean {
-            return binarySearchTypes(types, type) >= 0;
+            return binarySearch(types, type, getTypeId, compareValues) >= 0;
         }
 
         // Return true if the given intersection type contains (a) more than one unit type or (b) an object
@@ -7391,7 +7387,7 @@ namespace ts {
                 if (flags & TypeFlags.Number) typeSet.containsNumber = true;
                 if (flags & TypeFlags.StringOrNumberLiteral) typeSet.containsStringOrNumberLiteral = true;
                 const len = typeSet.length;
-                const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearchTypes(typeSet, type);
+                const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearch(typeSet, type, getTypeId, compareValues);
                 if (index < 0) {
                     if (!(flags & TypeFlags.Object && (<ObjectType>type).objectFlags & ObjectFlags.Anonymous &&
                         type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method) && containsIdenticalType(typeSet, type))) {
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 0ccb7253331..dd30b36470f 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1017,15 +1017,15 @@ namespace ts {
         while (low <= high) {
             const middle = low + ((high - low) >> 1);
             const midKey = keySelector(array[middle]);
-
-            if (keyComparer(midKey, key) === 0) {
-                return middle;
-            }
-            else if (keyComparer(midKey, key) > 0) {
-                high = middle - 1;
-            }
-            else {
-                low = middle + 1;
+            switch (keyComparer(midKey, key)) {
+                case Comparison.LessThan:
+                    low = middle + 1;
+                    break;
+                case Comparison.EqualTo:
+                    return middle;
+                case Comparison.GreaterThan:
+                    high = middle - 1;
+                    break;
             }
         }
 
@@ -1852,27 +1852,8 @@ namespace ts {
         return text1 ? Comparison.GreaterThan : Comparison.LessThan;
     }
 
-    export function sortAndDeduplicateDiagnostics(diagnostics: Diagnostic[]): Diagnostic[] {
-        return deduplicateSortedDiagnostics(diagnostics.sort(compareDiagnostics));
-    }
-
-    export function deduplicateSortedDiagnostics(diagnostics: Diagnostic[]): Diagnostic[] {
-        if (diagnostics.length < 2) {
-            return diagnostics;
-        }
-
-        const newDiagnostics = [diagnostics[0]];
-        let previousDiagnostic = diagnostics[0];
-        for (let i = 1; i < diagnostics.length; i++) {
-            const currentDiagnostic = diagnostics[i];
-            const isDupe = compareDiagnostics(currentDiagnostic, previousDiagnostic) === Comparison.EqualTo;
-            if (!isDupe) {
-                newDiagnostics.push(currentDiagnostic);
-                previousDiagnostic = currentDiagnostic;
-            }
-        }
-
-        return newDiagnostics;
+    export function sortAndDeduplicateDiagnostics(diagnostics: ReadonlyArray<Diagnostic>): Diagnostic[] {
+        return sortAndDeduplicate(diagnostics, compareDiagnostics);
     }
 
     export function normalizeSlashes(path: string): string {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 1ade3cc6374..4e55ae04d03 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -2291,6 +2291,7 @@ namespace ts {
         let nonFileDiagnostics: Diagnostic[] = [];
         const fileDiagnostics = createMap<Diagnostic[]>();
 
+        let hasReadNonFileDiagnostics = false;
         let diagnosticsModified = false;
         let modificationCount = 0;
 
@@ -2320,6 +2321,12 @@ namespace ts {
                 }
             }
             else {
+                // If we've already read the non-file diagnostics, do not modify the existing array.
+                if (hasReadNonFileDiagnostics) {
+                    hasReadNonFileDiagnostics = false;
+                    nonFileDiagnostics = nonFileDiagnostics.slice();
+                }
+
                 diagnostics = nonFileDiagnostics;
             }
 
@@ -2330,6 +2337,7 @@ namespace ts {
 
         function getGlobalDiagnostics(): Diagnostic[] {
             sortAndDeduplicate();
+            hasReadNonFileDiagnostics = true;
             return nonFileDiagnostics;
         }
 
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index d88f0e0854e..7ecf783aee6 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -531,15 +531,31 @@ namespace FourSlash {
             }
 
             for (const { start, length, messageText, file } of errors) {
-                Harness.IO.log("  from: " + showPosition(file, start) +
-                    ", to: " + showPosition(file, start + length) +
+                Harness.IO.log("  " + this.formatRange(file, start, length) +
                     ", message: " + ts.flattenDiagnosticMessageText(messageText, Harness.IO.newLine()) + "\n");
             }
+        }
 
-            function showPosition(file: ts.SourceFile, pos: number) {
+        private formatRange(file: ts.SourceFile, start: number, length: number) {
+            if (file) {
+                return `from: ${this.formatLineAndCharacterOfPosition(file, start)}, to: ${this.formatLineAndCharacterOfPosition(file, start + length)}`;
+            }
+            return "global";
+        }
+
+        private formatLineAndCharacterOfPosition(file: ts.SourceFile, pos: number) {
+            if (file) {
                 const { line, character } = ts.getLineAndCharacterOfPosition(file, pos);
                 return `${line}:${character}`;
             }
+            return "global";
+        }
+
+        private formatPosition(file: ts.SourceFile, pos: number) {
+            if (file) {
+                return file.fileName + "@" + pos;
+            }
+            return "global";
         }
 
         public verifyNoErrors() {
@@ -549,7 +565,7 @@ namespace FourSlash {
                 if (errors.length) {
                     this.printErrorLog(/*expectErrors*/ false, errors);
                     const error = errors[0];
-                    this.raiseError(`Found an error: ${error.file.fileName}@${error.start}: ${error.messageText}`);
+                    this.raiseError(`Found an error: ${this.formatPosition(error.file, error.start)}: ${error.messageText}`);
                 }
             });
         }
diff --git a/tests/cases/fourslash/incrementalParsingDynamicImport1.ts b/tests/cases/fourslash/incrementalParsingDynamicImport1.ts
index 0f15dcb3b1b..511f3835f5b 100644
--- a/tests/cases/fourslash/incrementalParsingDynamicImport1.ts
+++ b/tests/cases/fourslash/incrementalParsingDynamicImport1.ts
@@ -7,11 +7,11 @@
 
 //// var x1 = import("./foo");
 //// x1.then(foo => {
-////    var s: string = foo.bar();    
+////    var s: string = foo.bar();
 //// })
 //// /*1*/
 
-verify.numberOfErrorsInCurrentFile(1);
+verify.numberOfErrorsInCurrentFile(2);
 goTo.marker("1");
 edit.insert("  ");
-verify.numberOfErrorsInCurrentFile(1);
\ No newline at end of file
+verify.numberOfErrorsInCurrentFile(2);
\ No newline at end of file
diff --git a/tests/cases/fourslash/incrementalParsingDynamicImport3.ts b/tests/cases/fourslash/incrementalParsingDynamicImport3.ts
index b3da4c5b538..64aca403ad9 100644
--- a/tests/cases/fourslash/incrementalParsingDynamicImport3.ts
+++ b/tests/cases/fourslash/incrementalParsingDynamicImport3.ts
@@ -11,4 +11,4 @@
 verify.numberOfErrorsInCurrentFile(0);
 goTo.marker("1");
 edit.insert("(");
-verify.numberOfErrorsInCurrentFile(2);
\ No newline at end of file
+verify.numberOfErrorsInCurrentFile(3);
\ No newline at end of file