fix(compiler): do not remove whitespace wrapping i18n expansions (#31962)

Similar to interpolation, we do not want to completely remove whitespace
nodes that are siblings of an expansion.

For example, the following template

```html
<div>
  <strong>items left<strong> {count, plural, =1 {item} other {items}}
</div>
```

was being collapsed to

```html
<div><strong>items left<strong>{count, plural, =1 {item} other {items}}</div>
```

which results in the text looking like

```
items left4
```

instead it should be collapsed to

```html
<div><strong>items left<strong> {count, plural, =1 {item} other {items}}</div>
```

which results in the text looking like

```
items left 4
```

---

**Analysis of the code and manual testing has shown that this does not cause
the generated ids to change, so there is no breaking change here.**

PR Close #31962

packages/compiler/src/ml_parser/html_whitespaces.ts

@@ -60,18 +60,20 @@ export class WhitespaceVisitor implements html.Visitor {
     }
 
     return new html.Element(
-        element.name, element.attrs, html.visitAll(this, element.children), element.sourceSpan,
-        element.startSourceSpan, element.endSourceSpan, element.i18n);
+        element.name, element.attrs, visitAllWithSiblings(this, element.children),
+        element.sourceSpan, element.startSourceSpan, element.endSourceSpan, element.i18n);
   }
 
   visitAttribute(attribute: html.Attribute, context: any): any {
     return attribute.name !== PRESERVE_WS_ATTR_NAME ? attribute : null;
   }
 
-  visitText(text: html.Text, context: any): any {
+  visitText(text: html.Text, context: SiblingVisitorContext|null): any {
     const isNotBlank = text.value.match(NO_WS_REGEXP);
+    const hasExpansionSibling = context &&
+        (context.prev instanceof html.Expansion || context.next instanceof html.Expansion);
 
-    if (isNotBlank) {
+    if (isNotBlank || hasExpansionSibling) {
       return new html.Text(
           replaceNgsp(text.value).replace(WS_REPLACE_REGEXP, ' '), text.sourceSpan, text.i18n);
     }
@@ -91,3 +93,21 @@ export function removeWhitespaces(htmlAstWithErrors: ParseTreeResult): ParseTree
       html.visitAll(new WhitespaceVisitor(), htmlAstWithErrors.rootNodes),
       htmlAstWithErrors.errors);
 }
+
+interface SiblingVisitorContext {
+  prev: html.Node|undefined;
+  next: html.Node|undefined;
+}
+
+function visitAllWithSiblings(visitor: WhitespaceVisitor, nodes: html.Node[]): any[] {
+  const result: any[] = [];
+
+  nodes.forEach((ast, i) => {
+    const context: SiblingVisitorContext = {prev: nodes[i - 1], next: nodes[i + 1]};
+    const astResult = ast.visit(visitor, context);
+    if (astResult) {
+      result.push(astResult);
+    }
+  });
+  return result;
+}