Revert changing helpers to return References

Helpers are required to return `PathReference`s`, because their return
values can be later used in lookup positions, e.g.

```
{{#with (my-helper) as |foo|}}
  {{foo.bar}} <--- referenceFromMyHelper.get('bar')
{{/with}}
```

See emberjs/ember.js#13173 (comment) for more details.

We were previously using `Function.prototype.call` to invoke helpers,
which is typed to be `call(thisArg: any, args: any[]): any` which caused
us to not get a TypeError from the change (because the return value is
`any`). This commit fixed that as well.

packages/glimmer-runtime/lib/compiled/expressions/helper.ts

@@ -19,7 +19,8 @@ export default class CompiledHelper extends CompiledExpression<Opaque> {
   }
 
   evaluate(vm: VM): PathReference<Opaque> {
-    return this.helper.call(undefined, this.args.evaluate(vm));
+    let { helper } = this;
+    return helper(this.args.evaluate(vm));
   }
 
   toJSON(): string {

packages/glimmer-runtime/lib/environment.ts

@@ -204,7 +204,7 @@ type PositionalArguments = Opaque[];
 type KeywordArguments = Dict<Opaque>;
 
 export interface Helper {
-  (args: EvaluatedArgs): Reference<Opaque>;
+  (args: EvaluatedArgs): PathReference<Opaque>;
 }
 
 export interface ParsedStatement {

packages/glimmer-runtime/tests/updating-test.ts

@@ -1,6 +1,6 @@
-import { EvaluatedArgs, Template, RenderResult, SafeString } from "glimmer-runtime";
+import { EvaluatedArgs, Template, RenderResult, SafeString, ValueReference } from "glimmer-runtime";
 import { TestEnvironment, TestDynamicScope, equalTokens, stripTight } from "glimmer-test-helpers";
-import { PathReference, ConstReference } from "glimmer-reference";
+import { PathReference } from "glimmer-reference";
 import { UpdatableReference } from "glimmer-object-reference";
 import { Opaque } from "glimmer-util";
 
@@ -409,8 +409,8 @@ test("triple curlies with empty string initial value", assert => {
 test("double curlies with const SafeString", assert => {
   let rawString = '<b>bold</b> and spicy';
 
-  env.setHelper('const-foobar', (args: EvaluatedArgs) => {
-    return new ConstReference<Opaque>(makeSafeString(rawString));
+  env.registerInternalHelper('const-foobar', (args: EvaluatedArgs) => {
+    return new ValueReference<Opaque>(makeSafeString(rawString));
   });
 
   let template = compile('<div>{{const-foobar}}</div>');
@@ -430,8 +430,8 @@ test("double curlies with const SafeString", assert => {
 test("double curlies with const Node", assert => {
   let rawString = '<b>bold</b> and spicy';
 
-  env.setHelper('const-foobar', (args: EvaluatedArgs) => {
-    return new ConstReference<Opaque>(document.createTextNode(rawString));
+  env.registerInternalHelper('const-foobar', (args: EvaluatedArgs) => {
+    return new ValueReference<Opaque>(document.createTextNode(rawString));
   });
 
   let template = compile('<div>{{const-foobar}}</div>');
@@ -451,8 +451,8 @@ test("double curlies with const Node", assert => {
 test("triple curlies with const SafeString", assert => {
   let rawString = '<b>bold</b> and spicy';
 
-  env.setHelper('const-foobar', (args: EvaluatedArgs) => {
-    return new ConstReference<Opaque>(makeSafeString(rawString));
+  env.registerInternalHelper('const-foobar', (args: EvaluatedArgs) => {
+    return new ValueReference<Opaque>(makeSafeString(rawString));
   });
 
   let template = compile('<div>{{{const-foobar}}}</div>');
@@ -472,8 +472,8 @@ test("triple curlies with const SafeString", assert => {
 test("triple curlies with const Node", assert => {
   let rawString = '<b>bold</b> and spicy';
 
-  env.setHelper('const-foobar', (args: EvaluatedArgs) => {
-    return new ConstReference<Opaque>(document.createTextNode(rawString));
+  env.registerInternalHelper('const-foobar', (args: EvaluatedArgs) => {
+    return new ValueReference<Opaque>(document.createTextNode(rawString));
   });
 
   let template = compile('<div>{{{const-foobar}}}</div>');

packages/glimmer-test-helpers/lib/environment.ts

@@ -409,6 +409,10 @@ class HelperReference implements Reference<Opaque> {
 
     return helper(positional.value(), named.value());
   }
+
+  get(prop: InternedString): SimplePathReference<Opaque> {
+    return new SimplePathReference(this, prop);
+  }
 }
 
 export class TestEnvironment extends Environment {
@@ -426,7 +430,7 @@ export class TestEnvironment extends Environment {
     this.helpers[name] = (args: EvaluatedArgs) => new HelperReference(helper, args);
   }
 
-  setHelper(name: string, helper: GlimmerHelper) {
+  registerInternalHelper(name: string, helper: GlimmerHelper) {
     this.helpers[name] = helper;
   }