feat(compiler): Added "strictMetadataEmit" option to ngc (#10951)

ngc can now validate metadata before emitting to verify it doesn't
contain an error symbol that will result in a runtime error if
it is used by the StaticReflector.

To enable this add the section,

  "angularCompilerOptions": {
    "strictMetadataEmit": true
  }

to the top level of the tsconfig.json file passed to ngc.

Enabled metadata validation for packages that are intended to be
used statically.

modules/@angular/compiler-cli/src/static_reflector.ts

@@ -572,13 +572,13 @@ function expandedMessage(error: any): string {
   switch (error.message) {
     case 'Reference to non-exported class':
       if (error.context && error.context.className) {
-        return `Reference to a non-exported class ${error.context.className}`;
+        return `Reference to a non-exported class ${error.context.className}. Consider exporting the class`;
       }
       break;
     case 'Variable not initialized':
-      return 'Only initialized variables and constants can be referenced';
+      return 'Only initialized variables and constants can be referenced because the value of this variable is needed by the template compiler';
     case 'Destructuring not supported':
-      return 'Referencing an exported destructured variable or constant is not supported';
+      return 'Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring';
     case 'Could not resolve type':
       if (error.context && error.context.typeName) {
         return `Could not resolve type ${error.context.typeName}`;

modules/@angular/compiler-cli/test/reflector_host_spec.ts

@@ -38,6 +38,7 @@ describe('reflector_host', () => {
           genDir: '/tmp/project/src/gen/',
           basePath: '/tmp/project/src',
           skipMetadataEmit: false,
+          strictMetadataEmit: false,
           skipTemplateCodegen: false,
           trace: false
         },
@@ -47,6 +48,7 @@ describe('reflector_host', () => {
           genDir: '/tmp/project/gen',
           basePath: '/tmp/project/src/',
           skipMetadataEmit: false,
+          strictMetadataEmit: false,
           skipTemplateCodegen: false,
           trace: false
         },