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sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_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.
 
 part of dart2js.js_emitter;
 
 /**
  * Generates the code for all used classes in the program. Static fields (even
  * in classes) are ignored, since they can be treated as non-class elements.
  *
@@ skipping 1530 matching lines @@
 
   String buildSourceMap(CodeBuffer buffer, SourceFile compiledFile) {
     SourceMapBuilder sourceMapBuilder =
         new SourceMapBuilder(compiler.sourceMapUri);
     buffer.forEachSourceLocation(sourceMapBuilder.addMapping);
     return sourceMapBuilder.build(compiledFile);
   }
 
   void outputSourceMap(String code, String name) {
     if (!generateSourceMap) return;
-    SourceFile compiledFile = new SourceFile(null, compiler.assembledCode);
+    // Create a source file for the compilation output. This allows using
+    // [:getLine:] to transform offsets to line numbers in [SourceMapBuilder].
+    SourceFile compiledFile =
+        new StringSourceFile(null, compiler.assembledCode);
     String sourceMap = buildSourceMap(mainBuffer, compiledFile);
     compiler.outputProvider(name, 'js.map')
         ..add(sourceMap)
         ..close();
   }
 
   bool isDeferred(Element element) {
     return compiler.deferredLoadTask.isDeferred(element);
   }
 
   bool get areAnyElementsDeferred {
     return compiler.deferredLoadTask.areAnyElementsDeferred;
   }
 }