fix(@ngtools/webpack): import as results in the alias being undefined with Typescript 3.2

When using the `specifier.propertyName` with `typeChecker.getSymbolAtLocation` it will return a more detailed symbol then we originally have in the `usedSymbols` set.

We should probably use `symbol.id` to actually check if the symbols are the same, however the `id` is not exposed in the Symbol interface.

Using `node.name` will return the same symbol that we have stored in the set.

Fixes angular#13212

Author: Alan Agius

src/transformers/elide_imports.ts

@@ -100,7 +100,7 @@ export function elideImports(
       // "import { XYZ, ... } from 'abc';"
       const specifierOps = [];
       for (const specifier of node.importClause.namedBindings.elements) {
-        if (isUnused(specifier.propertyName || specifier.name)) {
+        if (isUnused(specifier.name)) {
           specifierOps.push(new RemoveNodeOperation(sourceFile, specifier));
         }
       }