Generate dynamic type errors according to spec.

runtime/lib/error.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.
 // Errors are created and thrown by DartVM only.
 // Changes here should also be reflected in corelib/error.dart as well
 
 class AssertionError {
   factory AssertionError._uninstantiable() {
     throw const UnsupportedOperationException(
         "AssertionError can only be allocated by the VM");
@@ skipping 15 matching lines @@
     throw const UnsupportedOperationException(
         "TypeError can only be allocated by the VM");
   }
   static _throwNew(int location,
                    Object src_value,
                    String dst_type_name,
                    String dst_name,
                    String malformed_error)
       native "TypeError_throwNew";
   String toString() {
-    if (malformedError != null) {
-      return malformedError;
+    String str = (malformedError != null) ? malformedError : "";
+    if ((dstName != null) && (dstName.length > 0)) {
+      str = str +
+          "type '$srcType' is not assignable to type '$dstType' of '$dstName'.";
+    } else {
+      str = str + "malformed type used in type test.";
     }
-    return "'$url': Failed type check: line $line pos $column: " +
-        "type '$srcType' is not assignable to type '$dstType' of '$dstName'.";
+    return str;
   }
   final String srcType;
   final String dstType;
   final String dstName;
   final String malformedError;
 }
 
 class FallThroughError {
   factory FallThroughError._uninstantiable() {
     throw const UnsupportedOperationException(
@@ skipping 23 matching lines @@
   String toString() => "Unresolved static method: url '$url' line $line " +
       "pos $column\n$failedResolutionLine\n";
 
   static _throwNew(int token_pos) native "StaticResolutionException_throwNew";
 
   final String failedResolutionLine;
   final String url;
   final int line;
   final int column;
 }