Add support for resolving symbolic links

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index aeebf4b2c808bad18a4fe7d55c77058848611a52..d179d7f523184133448de9af85162d5f6a82ae20 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -30,6 +30,7 @@ class MemoryResourceProvider implements ResourceProvider {
   int nextStamp = 0;
 
   final Context _pathContext;
+
   @override
   final AbsolutePathContext absolutePathContext;
 
@@ -309,6 +310,11 @@ class _MemoryDummyLink extends _MemoryResource implements File {
   }
 
   @override
+  File resolveSymbolicLinksSync() {
+    return throw new FileSystemException(path, "File does not exist");
+  }
+
+  @override
   Uri toUri() => new Uri.file(path, windows: _provider.pathContext == windows);
 
   @override
@@ -381,6 +387,9 @@ class _MemoryFile extends _MemoryResource implements File {
   }
 
   @override
+  File resolveSymbolicLinksSync() => this;
+
+  @override
   Uri toUri() => new Uri.file(path, windows: _provider.pathContext == windows);
 
   @override