Issue 3531. Don't allow 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 dd85e0b109ce94f4687c1b3210255d64f2e9e1f0..0639e82754e44d1d9b8c7aab1822e5e9684ce8cd 100644
--- a/compiler/java/com/google/dart/compiler/DartCompiler.java
+++ b/compiler/java/com/google/dart/compiler/DartCompiler.java
@@ -620,6 +620,15 @@ public class DartCompiler {
           continue;
         }
 
+        // Validate import prefixes.
+        for (LibraryNode importNode : lib.getImportPaths()) {
+          String prefix = importNode.getPrefix();
+          if (DartParser.PSEUDO_KEYWORDS_SET.contains(prefix)) {
+            context.onError(new DartCompilationError(importNode.getSourceInfo(),
+                ResolverErrorCode.BUILT_IN_IDENTIFIER_AS_IMPORT_PREFIX, prefix));
+          }
+        }
+
         // check that each imported library has a library directive
         for (LibraryUnit importedLib : lib.getImports()) {