Implement literal maps.

dart/frog/leg/lib/js_helper.dart

Index: dart/frog/leg/lib/js_helper.dart
diff --git a/dart/frog/leg/lib/js_helper.dart b/dart/frog/leg/lib/js_helper.dart
index a23ae9ba2ec725cc2a5302adc08698dfa47216d1..13b4c713241c2eefea0ae8e5b2aee9e549f92c91 100644
--- a/dart/frog/leg/lib/js_helper.dart
+++ b/dart/frog/leg/lib/js_helper.dart
@@ -314,9 +314,10 @@ checkGrowable(list, reason) {
 builtin$toString$0(var value) {
   if (JS('bool', @'typeof $0 == "object"', value)) {
     if (isJsArray(value)) {
-      return "Instance of 'List'";
+      return Collections.collectionToString(value);
+    } else {
+      return UNINTERCEPTED(value.toString());
     }
-    return UNINTERCEPTED(value.toString());
   }
   if (JS('bool', @'$0 === 0 && (1 / $0) < 0', value)) {
     return '-0.0';
@@ -1296,3 +1297,18 @@ unwrapException(ex) {
   }
   return ex;
 }
+
+/**
+ * Called by generated code to build a map literal. [keyValuePairs] is
+ * a list of key, value, key, value, ..., etc.
+ */
+makeLiteralMap(List keyValuePairs) {
+  Iterator iterator = keyValuePairs.iterator();
+  Map result = new LinkedHashMap();
+  while (iterator.hasNext()) {
+    String key = iterator.next();
+    var value = iterator.next();
+    result[key] = value;
+  }
+  return result;
+}