Add regression tests

Report unreachable code in JS files when --checkjs is passed, but not
otherwise.

tests/baselines/reference/unreachableJavascriptChecked.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/compiler/unreachable.js(3,5): error TS7027: Unreachable code detected.
+
+
+==== tests/cases/compiler/unreachable.js (1 errors) ====
+    function unreachable() {
+        return 1;
+        return 2;
+        ~~~~~~
+!!! error TS7027: Unreachable code detected.
+    }

tests/baselines/reference/unreachableJavascriptChecked.js

@@ -0,0 +1,11 @@
+//// [unreachable.js]
+function unreachable() {
+    return 1;
+    return 2;
+}
+
+//// [unreachable.js]
+function unreachable() {
+    return 1;
+    return 2;
+}

tests/baselines/reference/unreachableJavascriptUnchecked.js

@@ -0,0 +1,11 @@
+//// [unreachable.js]
+function unreachable() {
+    return 1;
+    return 2;
+}
+
+//// [unreachable.js]
+function unreachable() {
+    return 1;
+    return 2;
+}

tests/baselines/reference/unreachableJavascriptUnchecked.symbols

@@ -0,0 +1,7 @@
+=== tests/cases/compiler/unreachable.js ===
+function unreachable() {
+>unreachable : Symbol(unreachable, Decl(unreachable.js, 0, 0))
+
+    return 1;
+    return 2;
+}

tests/baselines/reference/unreachableJavascriptUnchecked.types

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/unreachable.js ===
+function unreachable() {
+>unreachable : () => 1 | 2
+
+    return 1;
+>1 : 1
+
+    return 2;
+>2 : 2
+}

tests/cases/compiler/unreachableJavascriptChecked.ts

@@ -0,0 +1,8 @@
+// @Filename: unreachable.js
+// @allowJs: true
+// @checkJs: true
+// @outDir: out
+function unreachable() {
+    return 1;
+    return 2;
+}

tests/cases/compiler/unreachableJavascriptUnchecked.ts

@@ -0,0 +1,8 @@
+// @Filename: unreachable.js
+// @allowJs: true
+// @checkJs: false
+// @outDir: out
+function unreachable() {
+    return 1;
+    return 2;
+}