Add Resource.delete() method.

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 f95d33ffc1bb44bcc8c197cdfd5cdad448e43e19..789095ceb8da8902315dc24fa79fef6a0fdb6d71 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -281,6 +281,11 @@ class _MemoryDummyLink extends _MemoryResource implements File {
   }
 
   @override
+  void delete() {
+    throw new FileSystemException(path, 'File could not be deleted');
+  }
+
+  @override
   bool isOrContains(String path) {
     return path == this.path;
   }
@@ -342,6 +347,11 @@ class _MemoryFile extends _MemoryResource implements File {
   }
 
   @override
+  void delete() {
+    _provider.deleteFile(path);
+  }
+
+  @override
   bool isOrContains(String path) {
     return path == this.path;
   }
@@ -488,6 +498,11 @@ class _MemoryFolder extends _MemoryResource implements Folder {
   }
 
   @override
+  void delete() {
+    _provider.deleteFolder(path);
+  }
+
+  @override
   Resource getChild(String relPath) {
     String childPath = canonicalizePath(relPath);
     _MemoryResource resource = _provider._pathToResource[childPath];