Issue 6971. Fixes for using built-in identifier as import prefix.

compiler/java/com/google/dart/compiler/DartCompiler.java

 // 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.
 
 package com.google.dart.compiler;
 
 import com.google.common.base.Objects;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Sets;
 import com.google.common.collect.Sets.SetView;
@@ skipping 589 matching lines @@
               includedSourceUris.add(uri);
             }
           }
         }
 
         // Validate imports.
         boolean hasIO = false;
         boolean hasHTML = false;
         for (LibraryNode importNode : lib.getImportPaths()) {
           String libSpec = importNode.getText();
-          String prefix = importNode.getPrefix();
           hasIO |= "dart:io".equals(libSpec);
           hasHTML |= "dart:html".equals(libSpec);
           // "dart:mirrors" are not done yet
           if ("dart:mirrors".equals(libSpec)) {
             context.onError(new DartCompilationError(importNode,
                 DartCompilerErrorCode.MIRRORS_NOT_FULLY_IMPLEMENTED));
           }
-          // validate import prefix
-          if (DartParser.PSEUDO_KEYWORDS_SET.contains(prefix)) {
-            context.onError(new DartCompilationError(importNode.getSourceInfo(),
-                ResolverErrorCode.BUILT_IN_IDENTIFIER_AS_IMPORT_PREFIX, prefix));
-          }
           // validate console/web mix
           if (hasIO && hasHTML) {
             context.onError(new DartCompilationError(importNode.getSourceInfo(),
                 DartCompilerErrorCode.CONSOLE_WEB_MIX));
           }
         }
 
         // check that each exported library has a library directive
         for (LibraryExport libraryExport : lib.getExports()) {
           LibraryUnit exportedLibrary = libraryExport.getLibrary();
@@ skipping 777 matching lines @@
           return br.readLine();
         } finally {
           br.close();
         }
       }
     } catch (Throwable e) {
     }
     return null;
   }
 }