diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 534bf85d119..86a68af1145 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -795,9 +795,12 @@ module Harness {
             }
         }
 
-        export function createSourceFileAndAssertInvariants(fileName: string, sourceText: string, languageVersion: ts.ScriptTarget) {
-            var result = ts.createSourceFile(fileName, sourceText, languageVersion, /*setParentNodes:*/ true);
-            Utils.assertInvariants(result, /*parent:*/ undefined);
+        export function createSourceFileAndAssertInvariants(fileName: string, sourceText: string, languageVersion: ts.ScriptTarget, assertInvariants = true) {
+            // Only set the parent nodes if we're asserting invariants.  We don't need them otherwise.
+            var result = ts.createSourceFile(fileName, sourceText, languageVersion, /*setParentNodes:*/ assertInvariants);
+            if (assertInvariants) {
+                Utils.assertInvariants(result, /*parent:*/ undefined);
+            }
             return result;
         }
 
@@ -805,7 +808,6 @@ module Harness {
         export var defaultLibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest);
         export var defaultES6LibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.es6.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest);
 
-
         // Cache these between executions so we don't have to re-parse them for every test
         export var fourslashFileName = 'fourslash.ts';
         export var fourslashSourceFile: ts.SourceFile;
@@ -926,7 +928,8 @@ module Harness {
                 settingsCallback?: (settings: ts.CompilerOptions) => void,
                 options?: ts.CompilerOptions,
                 // Current directory is needed for rwcRunner to be able to use currentDirectory defined in json file
-                currentDirectory?: string) {
+                currentDirectory?: string,
+                assertInvariants = true) {
 
                 options = options || { noResolve: false };
                 options.target = options.target || ts.ScriptTarget.ES3;
@@ -1074,7 +1077,7 @@ module Harness {
                 var register = (file: { unitName: string; content: string; }) => {
                     if (file.content !== undefined) {
                         var fileName = ts.normalizeSlashes(file.unitName);
-                        filemap[getCanonicalFileName(fileName)] = createSourceFileAndAssertInvariants(fileName, file.content, options.target);
+                        filemap[getCanonicalFileName(fileName)] = createSourceFileAndAssertInvariants(fileName, file.content, options.target, assertInvariants);
                     }
                 };
                 inputFiles.forEach(register);
diff --git a/src/harness/rwcRunner.ts b/src/harness/rwcRunner.ts
index b66322f8390..c1c3251cf02 100644
--- a/src/harness/rwcRunner.ts
+++ b/src/harness/rwcRunner.ts
@@ -90,7 +90,8 @@ module RWC {
                         /*settingsCallback*/ undefined, opts.options,
                         // Since all Rwc json file specified current directory in its json file, we need to pass this information to compilerHost
                         // so that when the host is asked for current directory, it should give the value from json rather than from process
-                        currentDirectory);
+                        currentDirectory,
+                        /*assertInvariants:*/ false);
                 });
 
                 function getHarnessCompilerInputUnit(fileName: string) {