Re-enable source maps

lib/src/codegen/js_printer.dart

 // Copyright (c) 2015, 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.
 
 library dev_compiler.src.codegen.js_printer;
 
 import 'dart:io' show Directory, File, Platform, Process;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:path/path.dart' as path;
@@ skipping 52 matching lines @@
 
   CompilationUnit unit;
   Uri uri;
 
   SourceMapPrintingContext(this.printer, this.outputDir);
 
   void emit(String string) {
     printer.add(string);
   }
 
+  AstNode _currentTopLevelDeclaration;
+
   void enterNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
-    if (node is CompilationUnit) {
-      unit = node;
+    if (node == null || node.offset == -1) return;
+    if (unit == null) {
+      // This is a top-level declaration.  Note: consecutive top-level
+      // declarations may come from different compilation units due to
+      // parts.
+      _currentTopLevelDeclaration = node;
+      unit = node.getAncestor((n) => n is CompilationUnit);
       uri = _makeRelativeUri(unit.element.source.uri);
-      return;
     }
-    if (unit == null || node == null || node.offset == -1) return;
 
+    assert(unit != null);
     var loc = _location(node.offset);
     var name = _getIdentifier(node);
     if (name != null) {
       // TODO(jmesserly): mark only uses the beginning of the span, but
       // we're required to pass this as a valid span.
       var end = _location(node.end);
       printer.mark(new SourceMapSpan(loc, end, name, isIdentifier: true));
     } else {
       printer.mark(loc);
     }
   }
 
   SourceLocation _location(int offset) =>
       locationForOffset(unit.lineInfo, uri, offset);
 
   Uri _makeRelativeUri(Uri src) {
     return new Uri(path: path.relative(src.path, from: outputDir));
   }
 
   void exitNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
-    if (node is CompilationUnit) {
-      unit = null;
-      uri = null;
-      return;
-    }
     if (unit == null || node == null || node.offset == -1) return;
 
     // TODO(jmesserly): in many cases marking the end will be unnecessary.
     printer.mark(_location(node.end));
+
+    if (_currentTopLevelDeclaration == node) {
+      unit = null;
+      uri = null;
+      _currentTopLevelDeclaration == null;
+      return;
+    }
   }
 
   String _getIdentifier(AstNode node) {
     if (node is SimpleIdentifier) return node.name;
     return null;
   }
 }