Use '--source-map' and '--out' options to support source maps from non-commandline.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 431 matching lines @@
         memberNodes[elementAsts[element].ast] = members;
       }
     }
 
     if (useMirrorHelperLibrary) {
       mirrorRenamer.addRenames(renames, topLevelNodes, collector);
     }
 
     final unparser = new EmitterUnparser(renames);
     emitCode(unparser, imports, topLevelNodes, memberNodes);
-    compiler.assembledCode = unparser.result;
+    String assembledCode = unparser.result;
+    compiler.outputProvider('', 'dart')
+        ..add(assembledCode)
+        ..close();
+    compiler.assembledCode = assembledCode;
 
     // Output verbose info about size ratio of resulting bundle to all
     // referenced non-platform sources.
     logResultBundleSizeInfo(topLevelElements);
   }
 
   void logResultBundleSizeInfo(Set<Element> topLevelElements) {
     Iterable<LibraryElement> referencedLibraries =
         compiler.libraries.values.where(isUserLibrary);
     // Sum total size of scripts in each referenced library.
@@ skipping 125 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);