Inject Blockly version into binaries (#2750)

* Inject the Blockly version in package.json into compressed Blockly builds.

core/blockly.js

@@ -65,6 +65,16 @@ goog.require('Blockly.Xml');
 var CLOSURE_DEFINES = {'goog.DEBUG': false};
 /* eslint-enable no-unused-vars */
 
+/**
+ * Blockly core version.
+ * This constant is overriden by the build script (build.py) to the value of the version
+ * in package.json. This is done during the gen_core build step.
+ * For local builds, you can pass --define='Blockly.VERSION=X.Y.Z' to the compiler
+ * to override this constant.
+ * @define {string}
+ */
+Blockly.VERSION = 'uncompiled';
+
 /**
  * The main workspace most recently used.
  * Set by Blockly.WorkspaceSvg.prototype.markFocused