Add some support for the code-point code-unit distinction.

sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart
index a9e49c0f3a94f4bd2e4135cc72eef5626c55527f..7738b57af2214acc80e440d8042b4735705fc2b0 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/constant_emitter.dart
@@ -87,23 +87,12 @@ class ConstantEmitter implements ConstantVisitor {
   /**
    * Write the contents of the quoted string to a [CodeBuffer] in
    * a form that is valid as JavaScript string literal content.
-   * The string is assumed quoted by single quote characters.
+   * The string is assumed quoted by double quote characters.
    */
-  void writeEscapedString(DartString string,
-                          CodeBuffer buffer,
-                          Node diagnosticNode) {
-    void onError(code) {
-      compiler.reportError(
-          diagnosticNode,
-          'Unhandled non-BMP character: U+${code.toRadixString(16)}');
-    }
-    writeJsonEscapedCharsOn(string.iterator(), buffer, onError);
-  }
-
   void visitString(StringConstant constant) {
-    buffer.add("'");
-    writeEscapedString(constant.value, buffer, constant.node);
-    buffer.add("'");
+    buffer.add('"');
+    writeJsonEscapedCharsOn(constant.value.slowToString(), buffer);
+    buffer.add('"');
   }
 
   void emitCanonicalVersion(Constant constant) {