Generate a dynamic type error when creating an instance with a bound error.

runtime/lib/error.dart

Index: runtime/lib/error.dart
===================================================================
--- runtime/lib/error.dart	(revision 5248)
+++ runtime/lib/error.dart	(working copy)
@@ -35,10 +35,10 @@
   String toString() {
     String str = (malformedError != null) ? malformedError : "";
     if ((dstName != null) && (dstName.length > 0)) {
-      str = str +
-          "type '$srcType' is not assignable to type '$dstType' of '$dstName'.";
+      str = "${str}type '$srcType' is not assignable "
+            "to type '$dstType' of '$dstName'.";
     } else {
-      str = str + "malformed type used in type test.";
+      str = "${str}malformed type used.";
     }
     return str;
   }