Emit a compile-time error when using doubles as switch case expressions.

sdk/lib/_internal/compiler/implementation/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 506 matching lines @@
       "Error: 'default' only allowed on last case of a switch.");
 
   static const MessageKind SWITCH_CASE_TYPES_NOT_EQUAL = const MessageKind(
       "Error: 'case' expressions do not all have type '#{type}'.");
 
   static const MessageKind SWITCH_CASE_TYPES_NOT_EQUAL_CASE = const MessageKind(
       "Info: 'case' expression of type '#{type}'.");
 
   static const MessageKind SWITCH_CASE_VALUE_OVERRIDES_EQUALS =
       const MessageKind(
-          "Error: 'case' expression value overrides 'operator=='.");
+          "Error: 'case' expression type overrides 'operator=='.");
 
   static const MessageKind INVALID_ARGUMENT_AFTER_NAMED = const MessageKind(
       "Error: Unnamed argument after named argument.");
 
   static const MessageKind NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
       "Error: Not a compile-time constant.");
 
   static const MessageKind CYCLIC_COMPILE_TIME_CONSTANTS = const MessageKind(
       "Error: Cycle in the compile-time constant computation.");
 
@@ skipping 892 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }