diff --git a/blocks/lists.js b/blocks/lists.js
index ced1e3203..8effd6a27 100644
--- a/blocks/lists.js
+++ b/blocks/lists.js
@@ -36,6 +36,7 @@ goog.require('Blockly');
 goog.require('Blockly.Blocks');
 goog.require('Blockly.FieldDropdown');
 goog.require('Blockly.FieldLabel');
+goog.require('Blockly.Mutator');
 
 
 /**
diff --git a/blocks/logic.js b/blocks/logic.js
index e36adb627..11ad00bda 100644
--- a/blocks/logic.js
+++ b/blocks/logic.js
@@ -36,6 +36,7 @@ goog.require('Blockly');
 goog.require('Blockly.Blocks');
 goog.require('Blockly.FieldDropdown');
 goog.require('Blockly.FieldLabel');
+goog.require('Blockly.Mutator');
 
 
 /**
diff --git a/blocks/procedures.js b/blocks/procedures.js
index 040636142..ca829a000 100644
--- a/blocks/procedures.js
+++ b/blocks/procedures.js
@@ -31,6 +31,7 @@ goog.require('Blockly.Blocks');
 goog.require('Blockly.FieldCheckbox');
 goog.require('Blockly.FieldLabel');
 goog.require('Blockly.FieldTextInput');
+goog.require('Blockly.Mutator');
 
 
 Blockly.Blocks['procedures_defnoreturn'] = {
diff --git a/blocks/text.js b/blocks/text.js
index 1dbc42f9f..8937b0e23 100644
--- a/blocks/text.js
+++ b/blocks/text.js
@@ -34,6 +34,7 @@ goog.require('Blockly.FieldImage');
 goog.require('Blockly.FieldMultilineInput');
 goog.require('Blockly.FieldTextInput');
 goog.require('Blockly.FieldVariable');
+goog.require('Blockly.Mutator');
 
 
 /**
diff --git a/build.py b/build.py
index c8a565f96..71ced9293 100755
--- a/build.py
+++ b/build.py
@@ -257,6 +257,7 @@ goog.provide('Blockly.FieldMultilineInput');
 goog.provide('Blockly.FieldNumber');
 goog.provide('Blockly.FieldTextInput');
 goog.provide('Blockly.FieldVariable');
+goog.provide('Blockly.Mutator');
 """))
     # Read in all the source files.
     filenames = glob.glob(os.path.join("blocks", "*.js"))
diff --git a/core/block.js b/core/block.js
index 8fee8d4a9..7f6a915f4 100644
--- a/core/block.js
+++ b/core/block.js
@@ -36,7 +36,6 @@ goog.require('Blockly.Events.BlockDelete');
 goog.require('Blockly.Events.BlockMove');
 goog.require('Blockly.Extensions');
 goog.require('Blockly.Input');
-goog.require('Blockly.Mutator');
 goog.require('Blockly.utils');
 goog.require('Blockly.utils.Coordinate');
 goog.require('Blockly.utils.object');
diff --git a/core/extensions.js b/core/extensions.js
index c15329559..938f7c684 100644
--- a/core/extensions.js
+++ b/core/extensions.js
@@ -33,7 +33,6 @@
  */
 goog.provide('Blockly.Extensions');
 
-goog.require('Blockly.Mutator');
 goog.require('Blockly.utils');
 
 
@@ -115,6 +114,9 @@ Blockly.Extensions.registerMutator = function(name, mixinObj, opt_helperFn,
   // Sanity checks passed.
   Blockly.Extensions.register(name, function() {
     if (hasMutatorDialog) {
+      if (!Blockly.Mutator) {
+        throw Error(errorPrefix + 'Missing require for Blockly.Mutator');
+      }
       this.setMutator(new Blockly.Mutator(opt_blockList));
     }
     // Mixin the object.
diff --git a/core/requires.js b/core/requires.js
index ed8849df5..b8acc0c4c 100644
--- a/core/requires.js
+++ b/core/requires.js
@@ -29,6 +29,7 @@ goog.provide('Blockly.requires');
 
 // Blockly Core.
 goog.require('Blockly');
+goog.require('Blockly.Mutator');
 goog.require('Blockly.Trashcan');
 goog.require('Blockly.ZoomControls');
 // Blockly Fields.
diff --git a/gulpfile.js b/gulpfile.js
index 2723b9bfc..8ee20a372 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -85,7 +85,7 @@ function prependHeader() {
 
 /**
  * Helper method for calling the Closure compiler.
- * @param {*} compilerOptions 
+ * @param {*} compilerOptions
  * @param {boolean=} opt_verbose Optional option for verbose logging
  */
 function compile(compilerOptions, opt_verbose) {
@@ -149,7 +149,8 @@ goog.provide('Blockly.FieldLabel');
 goog.provide('Blockly.FieldMultilineInput');
 goog.provide('Blockly.FieldNumber');
 goog.provide('Blockly.FieldTextInput');
-goog.provide('Blockly.FieldVariable');`;
+goog.provide('Blockly.FieldVariable');
+goog.provide('Blockly.Mutator');`;
   return gulp.src('blocks/*.js', {base: './'})
     // Add Blockly.Blocks to be compatible with the compiler.
     .pipe(gulp.replace(`goog.provide('Blockly.Constants.Colour');`,
@@ -700,7 +701,7 @@ gulp.task('package-node-core', function() {
  * A helper method for packaging a Blockly code generator into a UMD module.
  * @param {string} file Source file name.
  * @param {string} rename Destination file name.
- * @param {string} generator Generator export namespace. 
+ * @param {string} generator Generator export namespace.
  */
 function packageGenerator(file, rename, generator) {
   return gulp.src(file)