Add prefix constraints for deferred imports.

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 1183 matching lines @@
       "Hint: Did you forget a factory keyword here?");
 
   static const MessageKind DEFERRED_LIBRARY_NOT_FROM_MAIN =
       const MessageKind(
           "Info: DeferredLibrary used as an annotation here, "
           "but not used in the main library. Will not split the output.",
           howToFix:
             "Try adding a '@DeferredLibrary(...)' annotation in the main "
             "library");
 
+  static const MessageKind DEFERRED_LIBRARY_WITHOUT_PREFIX =
+      const MessageKind(
+          "Error: This import is deferred but there is no prefix keyword.",
+          howToFix:
+            "Try adding a prefix to the import.");
+
+  static const MessageKind DEFERRED_LIBRARY_DUPLICATE_PREFIX =
+      const MessageKind(
+          "Error: The prefix of this deferred import is not unique.",
+          howToFix:
+            "Try changing the import prefix.");
+
   static const MessageKind ILLEGAL_STATIC = const MessageKind(
       "Error: Modifier static is only allowed on functions declared in "
       "a class.");
 
   static const MessageKind STATIC_FUNCTION_BLOAT = const MessageKind(
       "Hint: Using '#{class}.#{name}' may lead to unnecessarily large "
       "generated code.",
       howToFix:
           "Try adding '@MirrorsUsed(...)' as described at "
           "https://goo.gl/Akrrog.");
@@ skipping 747 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);
 }