Add ability for the angular plugin to set ErrorVerifier.enclosingClass.

There is a similar setter on Resolver, and without it, users are seeing
an issue:
https://github.com/dart-lang/angular_analyzer_plugin/issues/567.

Alternative options to modifying pkg:analyzer APIs are few. One option
is to block UNQUALIFIED_STATIC_REFERNCE errors, however, that will be
too general. We would need to implement a lookup to double-check when
those errors are reported, if they should be.

Alternatively the plugin could block all of them, and then do a second
pass to provide them where the plugins deems they should exist.

This seems the easiest approach and mirrors the api that [Resolver.set
enclosingClass] provides, but the chain of assertions makes we want a
second look on it.

Change-Id: I12df71719c7a0e968a0b3a43cc27c69160ec5210
Reviewed-on: https://dart-review.googlesource.com/54703
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -209,6 +209,24 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
    */
   ClassElementImpl _enclosingClass;
 
+  ClassElement get enclosingClass => _enclosingClass;
+
+  /**
+   * For consumers of error verification as a library, (currently just the
+   * angular plugin), expose a setter that can make the errors reported more
+   * accurate when dangling code snippets are being resolved from a class
+   * context. Note that this setter is very defensive for potential misuse; it
+   * should not be modified in the middle of visiting a tree and requires an
+   * analyzer-provided Impl instance to work.
+   */
+  set enclosingClass(ClassElement classElement) {
+    assert(classElement is ClassElementImpl);
+    assert(_enclosingClass == null);
+    assert(_enclosingEnum == null);
+    assert(_enclosingFunction == null);
+    _enclosingClass = classElement;
+  }
+
   /**
    * The enum containing the AST nodes being visited, or `null` if we are not
    * in the scope of an enum.