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

compiler/java/com/google/dart/compiler/parser/Token.java

Index: compiler/java/com/google/dart/compiler/parser/Token.java
diff --git a/compiler/java/com/google/dart/compiler/parser/Token.java b/compiler/java/com/google/dart/compiler/parser/Token.java
index 59b754eca19213a9d024589e1cd4da9549f74da3..74e211d85255ec6c6544954f07bec11d51e6957d 100644
--- a/compiler/java/com/google/dart/compiler/parser/Token.java
+++ b/compiler/java/com/google/dart/compiler/parser/Token.java
@@ -179,6 +179,9 @@ public enum Token {
    */
   public static Token lookup(String syntax) {
     Token token = tokens.get(syntax);
+    if ("as".equals(syntax)) {
+      return IDENTIFIER;
+    }
     if (token == null) {
       return IDENTIFIER;
     }