diff --git a/compiler/generate/index.js b/compiler/generate/index.js
index fa2ba7816a..614668ae9e 100644
--- a/compiler/generate/index.js
+++ b/compiler/generate/index.js
@@ -238,7 +238,7 @@ export default function generate ( parsed, source, options ) {
 	const topLevelStatements = [];
 
 	const setStatements = [ deindent`
-		const oldState = state;
+		var oldState = state;
 		state = Object.assign( {}, oldState, newState );
 	` ];
 
@@ -392,19 +392,19 @@ export default function generate ( parsed, source, options ) {
 			var callbacks = Object.create( null );
 
 			function dispatchObservers ( group, newState, oldState ) {
-				for ( const key in group ) {
+				for ( var key in group ) {
 					if ( !( key in newState ) ) continue;
 
-					const newValue = newState[ key ];
-					const oldValue = oldState[ key ];
+					var newValue = newState[ key ];
+					var oldValue = oldState[ key ];
 
 					if ( newValue === oldValue && typeof newValue !== 'object' ) continue;
 
-					const callbacks = group[ key ];
+					var callbacks = group[ key ];
 					if ( !callbacks ) continue;
 
-					for ( let i = 0; i < callbacks.length; i += 1 ) {
-						const callback = callbacks[i];
+					for ( var i = 0; i < callbacks.length; i += 1 ) {
+						var callback = callbacks[i];
 						if ( callback.__calling ) continue;
 
 						callback.__calling = true;
@@ -431,32 +431,32 @@ export default function generate ( parsed, source, options ) {
 				${setStatements.join( '\n\n' )}
 			};
 
-			this.observe = function ( key, callback, options = {} ) {
-				const group = options.defer ? observers.deferred : observers.immediate;
+			this.observe = function ( key, callback, options ) {
+				var group = ( options && options.defer ) ? observers.deferred : observers.immediate;
 
 				( group[ key ] || ( group[ key ] = [] ) ).push( callback );
 
-				if ( options.init !== false ) {
+				if ( !options || options.init !== false ) {
 					callback.__calling = true;
 					callback.call( component, state[ key ] );
 					callback.__calling = false;
 				}
 
 				return {
-					cancel () {
-						const index = group[ key ].indexOf( callback );
+					cancel: function () {
+						var index = group[ key ].indexOf( callback );
 						if ( ~index ) group[ key ].splice( index, 1 );
 					}
 				};
 			};
 
 			this.on = function on ( eventName, handler ) {
-				const handlers = callbacks[ eventName ] || ( callbacks[ eventName ] = [] );
+				var handlers = callbacks[ eventName ] || ( callbacks[ eventName ] = [] );
 				handlers.push( handler );
 
 				return {
 					cancel: function () {
-						const index = handlers.indexOf( handler );
+						var index = handlers.indexOf( handler );
 						if ( ~index ) handlers.splice( index, 1 );
 					}
 				};
diff --git a/compiler/generate/visitors/EachBlock.js b/compiler/generate/visitors/EachBlock.js
index 474eabce64..1cdce184e9 100644
--- a/compiler/generate/visitors/EachBlock.js
+++ b/compiler/generate/visitors/EachBlock.js
@@ -48,7 +48,7 @@ export default {
 		` );
 
 		generator.current.teardownStatements.push( deindent`
-			for ( let i = 0; i < ${name}_iterations.length; i += 1 ) {
+			for ( var i = 0; i < ${name}_iterations.length; i += 1 ) {
 				${name}_iterations[i].teardown( detach );
 			}
 
diff --git a/compiler/generate/visitors/attributes/addElementAttributes.js b/compiler/generate/visitors/attributes/addElementAttributes.js
index cb81a9a1f6..8637266ffd 100644
--- a/compiler/generate/visitors/attributes/addElementAttributes.js
+++ b/compiler/generate/visitors/attributes/addElementAttributes.js
@@ -143,7 +143,7 @@ export default function addElementAttributes ( generator, node, local ) {
 
 			if ( attribute.name in generator.events ) {
 				local.init.push( deindent`
-					const ${handlerName} = template.events.${attribute.name}.call( component, ${local.name}, function ( event ) {
+					var ${handlerName} = template.events.${attribute.name}.call( component, ${local.name}, function ( event ) {
 						${handlerBody}
 					});
 				` );
diff --git a/test/test.js b/test/test.js
index 481df8478f..9d132849e3 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1,8 +1,10 @@
 import deindent from '../compiler/generate/utils/deindent.js';
+import spaces from '../compiler/utils/spaces.js';
 import assert from 'assert';
 import * as path from 'path';
 import * as fs from 'fs';
 import jsdom from 'jsdom';
+import * as acorn from 'acorn';
 
 import * as consoleGroup from 'console-group';
 consoleGroup.install();
@@ -241,6 +243,16 @@ describe( 'svelte', () => {
 					return `${i}: ${line.replace( /^\t+/, match => match.split( '\t' ).join( '    ' ) )}`;
 				}).join( '\n' );
 
+				// check that no ES2015+ syntax slipped in
+				try {
+					const startIndex = code.indexOf( 'function renderMainFragment' ); // may change!
+					const es5 = spaces( startIndex ) + code.slice( startIndex ).replace( /export default .+/, '' );
+					acorn.parse( es5, { ecmaVersion: 5 });
+				} catch ( err ) {
+					console.log( withLineNumbers ); // eslint-disable-line no-console
+					throw err;
+				}
+
 				cache[ path.resolve( `test/compiler/${dir}/main.html` ) ] = code;
 
 				let SvelteComponent;