Source maps - JSON strings need double quotes.

sdk/lib/_internal/compiler/implementation/source_map_builder.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library source_map_builder;
 
 import 'util/util.dart';
 import 'scanner/scannerlib.dart' show Token;
 import 'source_file.dart';
 
@@ skipping 39 matching lines @@
 
   void addMapping(int targetOffset, SourceFileLocation sourceLocation) {
     entries.add(new SourceMapEntry(sourceLocation, targetOffset));
   }
 
   void printStringListOn(List<String> strings, StringBuffer buffer) {
     bool first = true;
     buffer.add('[');
     for (String string in strings) {
       if (!first) buffer.add(',');
-      buffer.add("'");
+      buffer.add('"');
       writeJsonEscapedCharsOn(string.charCodes.iterator(), buffer, null);
-      buffer.add("'");
+      buffer.add('"');
       first = false;
     }
     buffer.add(']');
   }
 
   String build(SourceFile targetFile) {
     StringBuffer buffer = new StringBuffer();
     buffer.add('{\n');
     buffer.add('  "version": 3,\n');
     buffer.add('  "mappings": "');
@@ skipping 106 matching lines @@
 
   int getColumn() => sourceFile.getColumn(getLine(), token.charOffset);
 
   String getSourceName() {
     if (token.isIdentifier()) return token.slowToString();
     return null;
   }
 
   bool isValid() => token.charOffset < sourceFile.text.length;
 }