Fix mocha tests

tests/mocha/block_test.js

@@ -1657,6 +1657,8 @@ suite('Blocks', function() {
       });
       teardown(function() {
         this.workspace.dispose();
+        // Clear all registered themes.
+        Blockly.registry.typeMap_['theme'] = {};
       });
       test('Set colour hue', function() {
         this.block.setColour('20');

tests/mocha/theme_test.js

@@ -11,6 +11,10 @@
 'use strict';
 
 suite('Theme', function() {
+  teardown(function() {
+    // Clear all registered themes.
+    Blockly.registry.typeMap_['theme'] = {};
+  });
 
   function defineThemeTestBlocks() {
     Blockly.defineBlocksWithJsonArray([{