Don't do folder existence checks when parsing pub list results.

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 efb779ccd12447ce6b20ac32cd054bf7686a8193..6473875b7b85fafa37c4de1b840e94ef40ed2a78 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -174,6 +174,9 @@ class _MemoryFile extends _MemoryResource implements File {
   _MemoryFile(MemoryResourceProvider provider, String path)
       : super(provider, path);
 
+  @override
+  bool get exists => _provider._pathToResource[path] is _MemoryFile;
+
   int get modificationStamp {
     int stamp = _provider._pathToTimestamp[path];
     if (stamp == null) {
@@ -286,6 +289,7 @@ class _MemoryFileSource extends Source {
 class _MemoryFolder extends _MemoryResource implements Folder {
   _MemoryFolder(MemoryResourceProvider provider, String path)
       : super(provider, path);
+
   @override
   Stream<WatchEvent> get changes {
     StreamController<WatchEvent> streamController =
@@ -304,6 +308,9 @@ class _MemoryFolder extends _MemoryResource implements Folder {
   }
 
   @override
+  bool get exists => _provider._pathToResource[path] is _MemoryFolder;
+
+  @override
   String canonicalizePath(String relPath) {
     relPath = posix.normalize(relPath);
     String childPath = posix.join(path, relPath);
@@ -327,6 +334,16 @@ class _MemoryFolder extends _MemoryResource implements Folder {
   }
 
   @override
+  _MemoryFolder getChildAssumingFolder(String relPath) {
+    String childPath = canonicalizePath(relPath);
+    _MemoryResource resource = _provider._pathToResource[childPath];
+    if (resource is _MemoryFolder) {
+      return resource;
+    }
+    return new _MemoryFolder(_provider, childPath);
+  }
+
+  @override
   List<Resource> getChildren() {
     List<Resource> children = <Resource>[];
     _provider._pathToResource.forEach((resourcePath, resource) {
@@ -357,9 +374,6 @@ abstract class _MemoryResource implements Resource {
   _MemoryResource(this._provider, this.path);
 
   @override
-  bool get exists => _provider._pathToResource.containsKey(path);
-
-  @override
   get hashCode => path.hashCode;
 
   @override