Handle imports of dart:core and dart:coreimpl

dart/frog/leg/scanner/scanner_task.dart

 // 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.
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name() => 'Scanner';
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
@@ skipping 25 matching lines @@
         }
       } else if (tag.isSource()) {
         Script script = compiler.readScript(resolved, tag);
         CompilationUnitElement unit =
           new CompilationUnitElement(script, library);
         compiler.withCurrentElement(unit, () => scan(unit));
       } else {
         compiler.cancel("illegal script tag: ${tag.tag}", node: tag);
       }
     }
+    // TODO(ahe): During Compiler.scanBuiltinLibraries,
+    // compiler.coreLibrary is null. Clean this up when there is a
+    // better way to access "dart:core".
+    bool implicitlyImportCoreLibrary = compiler.coreLibrary !== null;
     for (ScriptTag tag in imports.toLink()) {
       // Now that we have processed all the source tags, it is safe to
       // start loading other libraries.
       SourceString argument = tag.argument.value.copyWithoutQuotes(1, 1);
       Uri resolved = base.resolve(argument.toString());
+      if (resolved.toString() == "dart:core") {
+        implicitlyImportCoreLibrary = false;
+      }
       importLibrary(library, loadLibrary(resolved, tag), tag.prefix);
     }
-    if (compiler.coreLibrary !== null) {
+    if (implicitlyImportCoreLibrary) {
       importLibrary(library, compiler.coreLibrary, null);
     }
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     compiler.log("scanning $compilationUnit");
     Script script = compilationUnit.script;
     Token tokens;
     try {
       tokens = new StringScanner(script.text).tokenize();
@@ skipping 34 matching lines @@
     } else {
       for (Link<Element> link = imported.topLevelElements; !link.isEmpty();
            link = link.tail) {
         compiler.withCurrentElement(link.head, () {
             library.define(link.head, compiler);
           });
       }
     }
   }
 }