Optimize the AST transform for top-level elements

Don’t bother to convert top-level elements into dynamic calls if they
aren’t the top-level element *of a template*.

Consider a template like this:

```
{{#each posts as |post|}}
<div>hello</div>
{{/each}}
```

The previous version of the transform would convert the nested template
(the <div>) into a dynamic call, because the AST transformation didn’t
differentiate between top-level templates and nested templates. It
didn’t have any other effects (the dynamic call is required to restore
the semantics of the original static form) but it unnecessarily bloats
templates and makes execution slower.

packages/ember-template-compiler/lib/plugins/transform-top-level-components.js

@@ -13,7 +13,7 @@ function TransformTopLevelComponents() {
 TransformTopLevelComponents.prototype.transform = function TransformTopLevelComponents_transform(ast) {
   let b = this.syntax.builders;
 
-  hasSingleComponentNode(ast.body, component => {
+  hasSingleComponentNode(ast, component => {
     if (component.type === 'ComponentNode') {
       component.tag = `@${component.tag}`;
       component.isStatic = true;
@@ -35,7 +35,10 @@ TransformTopLevelComponents.prototype.transform = function TransformTopLevelComp
   return ast;
 };
 
-function hasSingleComponentNode(body, componentCallback, elementCallback) {
+function hasSingleComponentNode(program, componentCallback, elementCallback) {
+  let { loc, body } = program;
+  if (!loc || loc.start.line !== 1 || loc.start.column !== 0) { return; }
+
   let lastComponentNode;
   let lastIndex;
   let nodeCount = 0;