Source maps - JSON strings need double quotes.

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

Index: sdk/lib/_internal/compiler/implementation/source_map_builder.dart
diff --git a/sdk/lib/_internal/compiler/implementation/source_map_builder.dart b/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
index 06a6ca02c00852f85c8321ed5169aafc729da4da..98a1a01434c287867b67b80ef3cf7f0091f22233 100644
--- a/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
+++ b/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
@@ -57,9 +57,9 @@ class SourceMapBuilder {
     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(']');