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

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

Index: compiler/java/com/google/dart/compiler/DartCompiler.java
diff --git a/compiler/java/com/google/dart/compiler/DartCompiler.java b/compiler/java/com/google/dart/compiler/DartCompiler.java
index 5deca30837677caf68962202055b0d3ce87d6c2c..9e1995da296ca039a928554a55d5c3a9f790d2c1 100644
--- a/compiler/java/com/google/dart/compiler/DartCompiler.java
+++ b/compiler/java/com/google/dart/compiler/DartCompiler.java
@@ -607,7 +607,6 @@ public class DartCompiler {
         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
@@ -615,11 +614,6 @@ public class DartCompiler {
             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(),