Diagnose illegal lvalues better.

dart/tests/language/syntax_test.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.
 
 class SyntaxTest {
   // "this" cannot be used as a field name.
   SyntaxTest this; /// 01: compile-time error
 
   // Syntax error.
   foo {} /// 02: compile-time error
@@ skipping 215 matching lines @@
     G<> t; /// 57: compile-time error
     G<null> t; /// 58: compile-time error
     A<void> a = null; /// 59: compile-time error
     void v; /// 60: compile-time error
     void v = null; /// 61: compile-time error
     print(null is void); /// 62: compile-time error
     new B();
 
     new Bad();
 
+    1 + 2 = 1; /// 63: compile-time error
+    new SyntaxTest() = 1; /// 64: compile-time error
+    futureOf(null) = 1; /// 65: compile-time error
+
   } catch (ex) {
     // Swallowing exceptions. Any error should be a compile-time error
     // which kills the current isolate.
   }
 }
 
 class Bad {
   factory Bad<Bad(String type) { return null; } /// 63: compile-time error
 }