Cache path -> URI mapping in SourceMapPrintingContext.

pkg/dev_compiler/lib/src/compiler/source_map_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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:source_maps/source_maps.dart' hide Printer;
 import 'package:source_span/source_span.dart' show SourceLocation;
 
 import '../js_ast/js_ast.dart' as JS;
 
 class SourceMapPrintingContext extends JS.SimpleJavaScriptPrintingContext {
   /// Current line in the buffer;
   int _line = 0;
 
   /// Current column in the buffer.
   int _column = 0;
 
   /// The source_maps builder we write JavaScript code to.
   final sourceMap = new SourceMapBuilder();
 
+  /// The cache of URIs for paths.
+  final _sourceUrlCache = <String, Object>{};
+
   CompilationUnit unit;
   String sourcePath;
   AstNode _currentTopLevelDeclaration;
 
   @override
   void emit(String code) {
     var chars = code.runes.toList();
     var length = chars.length;
     for (int i = 0; i < length; i++) {
       var c = chars[i];
@@ skipping 58 matching lines @@
 
   void _mark(int offset, [String identifier]) {
     var loc = unit.lineInfo.getLocation(offset);
     // Chrome Devtools wants a mapping for the beginning of
     // a line, so bump locations at the end of a line to the beginning of
     // the next line.
     var next = unit.lineInfo.getLocation(offset + 1);
     if (next.lineNumber == loc.lineNumber + 1) {
       loc = next;
     }
-    var sourceUrl =
-        sourcePath.startsWith('dart:') || sourcePath.startsWith('package:')
-            ? sourcePath
-            : new Uri.file(sourcePath);
+    var sourceUrl = _sourceUrlCache.putIfAbsent(
+        sourcePath,
+        () =>
+            sourcePath.startsWith('dart:') || sourcePath.startsWith('package:')
+                ? sourcePath
+                : new Uri.file(sourcePath));
     sourceMap.addLocation(
         new SourceLocation(offset,
             sourceUrl: sourceUrl,
             line: loc.lineNumber - 1,
             column: loc.columnNumber - 1),
         new SourceLocation(buffer.length, line: _line, column: _column),
         identifier);
   }
 }
 
 const int _LF = 10;
 const int _CR = 13;