Check for ClassDeclaration in getBaseTypeNodeOfClass

Previously, it just used valueDeclaration. Now, it searches the
declarations.

src/compiler/checker.ts

@@ -2766,7 +2766,10 @@ namespace ts {
         }
 
         function getBaseTypeNodeOfClass(type: InterfaceType): ExpressionWithTypeArguments {
-            return getClassExtendsHeritageClauseElement(<ClassLikeDeclaration>type.symbol.valueDeclaration);
+            let classDeclaration = getDeclarationOfKind(type.symbol, SyntaxKind.ClassDeclaration);
+            if (classDeclaration) {
+                return getClassExtendsHeritageClauseElement(classDeclaration as ClassLikeDeclaration);
+            }
         }
 
         function getConstructorsForTypeArguments(type: ObjectType, typeArgumentNodes: TypeNode[]): Signature[] {