Move dart2js unittests to subfolders

* All helpers in /helpers/
* Tests of user-facing behavior in /end_to_end/
* Tests of internal models and unittests of internal classes in /model/

Change-Id: Ic7a7e58aa01f92f8572514ae81230804f36af67e
Reviewed-on: https://dart-review.googlesource.com/72644
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

Author: johnniwinther

tests/compiler/dart2js/codegen/arithmetic_simplification_test.dart

@@ -4,7 +4,7 @@
 // Test constant folding on numbers.
 
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String INT_PLUS_ZERO = """
 int foo(int x) => x;

tests/compiler/dart2js/codegen/array_static_intercept_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo(a) {

tests/compiler/dart2js/codegen/builtin_equals_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST = r"""
 foo() {

tests/compiler/dart2js/codegen/builtin_interceptor_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo() {

tests/compiler/dart2js/codegen/class_codegen2_test.dart

@@ -6,7 +6,7 @@
 import 'dart:async';
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A { foo() => 499; }

tests/compiler/dart2js/codegen/class_codegen_test.dart

@@ -5,7 +5,7 @@
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A { }

tests/compiler/dart2js/codegen/class_order_test.dart

@@ -5,7 +5,7 @@
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A { foo() => 499; }

tests/compiler/dart2js/codegen/closure_codegen_test.dart

@@ -6,7 +6,7 @@
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_INVOCATION0 = r"""
 main() {

tests/compiler/dart2js/codegen/code_motion_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo(int a, int b, bool param2) {

tests/compiler/dart2js/codegen/constant_folding_codeUnitAt_test.dart

@@ -4,7 +4,7 @@
 // Test constant folding on numbers.
 
 import 'package:async_helper/async_helper.dart';
-import '../compiler_helper.dart';
+import '../helpers/compiler_helper.dart';
 
 const String TEST_1 = """
 foo() {