Support enabling start hats from themes (#3722)

* Support themes configuring the start hats of blocks. Add test themes

tests/themes/test_themes.js

@@ -0,0 +1,62 @@
+/**
+ * @license
+ * Copyright 2017 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+'use strict';
+
+goog.provide('Blockly.TestThemes');
+
+
+/**
+ * A theme with classic colours but enables start hats.
+ */
+Blockly.Themes.TestHats = new Blockly.Theme('testhats',
+    Blockly.Themes.Classic.blockStyles, Blockly.Themes.Classic.categoryStyles);
+Blockly.Themes.TestHats.setStartHats(true);
+
+/**
+ * A theme with classic colours but a different font.
+ */
+Blockly.Themes.TestFont = new Blockly.Theme('testfont',
+    Blockly.Themes.Classic.blockStyles, Blockly.Themes.Classic.categoryStyles);
+Blockly.Themes.TestFont.setFontStyle({
+  'family': '"Times New Roman", Times, serif',
+  'weight': null, // Use default font-weight
+  'size': 16
+});
+
+/**
+ * Holds the test theme name to Theme instance mapping.
+ * @type {!Object<string, Blockly.Theme>}
+ * @private
+ */
+Blockly.Themes.testThemes_ = {
+  'Test Hats': Blockly.Themes.TestHats,
+  'Test Font': Blockly.Themes.TestFont
+};
+
+/**
+ * Get a test theme by name.
+ * @param {string} value The theme name.
+ * @return {Blockly.Theme} A theme object or undefined if one doesn't exist.
+ * @package
+ */
+function getTestTheme(value) {
+  return Blockly.Themes.testThemes_[value];
+}
+
+/**
+ * Populate the theme changer dropdown to list the set of test themes.
+ * @package
+ */
+function populateTestThemes() {
+  var themeChanger = document.getElementById('themeChanger');
+  var keys = Object.keys(Blockly.Themes.testThemes_);
+  for (var i = 0, key; (key = keys[i]); i++) {
+    var option = document.createElement('option');
+    option.setAttribute('value', key);
+    option.textContent = key;
+    themeChanger.appendChild(option);
+  }
+}