[refactor] kick some stuff out of js_codegen.dart

lib/src/codegen/js_printer.dart

Index: lib/src/codegen/js_printer.dart
diff --git a/lib/src/codegen/js_printer.dart b/lib/src/codegen/js_printer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..01dfb7544d520cc7777b109048f6c5eab841fa3b
--- /dev/null
+++ b/lib/src/codegen/js_printer.dart
@@ -0,0 +1,109 @@
+// 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;
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:path/path.dart' as path;
+import 'package:source_maps/source_maps.dart' as srcmaps show Printer;
+import 'package:source_maps/source_maps.dart' show SourceMapSpan;
+import 'package:source_span/source_span.dart' show SourceLocation;
+
+import 'package:dev_compiler/src/js/js_ast.dart' as JS;
+import 'package:dev_compiler/src/utils.dart'
+    show computeHash, locationForOffset;
+
+import 'js_names.dart' show JSNamer;
+
+String writeJsLibrary(JS.Program jsTree, String outputPath,
+    {bool emitSourceMaps: false}) {
+  new Directory(path.dirname(outputPath)).createSync(recursive: true);
+  if (emitSourceMaps) {
+    var outFilename = path.basename(outputPath);
+    var printer = new srcmaps.Printer(outFilename);
+    writeNodeToContext(jsTree,
+        new SourceMapPrintingContext(printer, path.dirname(outputPath)));
+    printer.add('//# sourceMappingURL=$outFilename.map');
+    // Write output file and source map
+    var text = printer.text;
+    new File(outputPath).writeAsStringSync(text);
+    new File('$outputPath.map').writeAsStringSync(printer.map);
+    return computeHash(text);
+  } else {
+    var text = jsNodeToString(jsTree);
+    new File(outputPath).writeAsStringSync(text);
+    return computeHash(text);
+  }
+}
+
+void writeNodeToContext(JS.Node node, JS.JavaScriptPrintingContext context) {
+  var opts = new JS.JavaScriptPrintingOptions(allowKeywordsInProperties: true);
+  node.accept(new JS.Printer(opts, context, localNamer: new JSNamer(node)));
+}
+
+String jsNodeToString(JS.Node node) {
+  var context = new JS.SimpleJavaScriptPrintingContext();
+  writeNodeToContext(node, context);
+  return context.getText();
+}
+
+class SourceMapPrintingContext extends JS.JavaScriptPrintingContext {
+  final srcmaps.Printer printer;
+  final String outputDir;
+
+  CompilationUnit unit;
+  Uri uri;
+
+  SourceMapPrintingContext(this.printer, this.outputDir);
+
+  void emit(String string) {
+    printer.add(string);
+  }
+
+  void enterNode(JS.Node jsNode) {
+    AstNode node = jsNode.sourceInformation;
+    if (node is CompilationUnit) {
+      unit = node;
+      uri = _makeRelativeUri(unit.element.source.uri);
+      return;
+    }
+    if (unit == null || node == null || node.offset == -1) return;
+
+    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, 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));
+  }
+
+  String _getIdentifier(AstNode node) {
+    if (node is SimpleIdentifier) return node.name;
+    return null;
+  }
+}