ts2.4: work around object spread with Partial

You can no longer update an object using an object spread with
Partial<T>.

Seee bug:
  microsoft/TypeScript#16509

Work around it for now.

Author: evmar

src/main.ts

@@ -159,7 +159,8 @@ export function toClosureJS(
     {jsFiles: Map<string, string>, externs: string}|null {
   const closureJSOptions: ClosureJSOptions = {
     ...getDefaultClosureJSOptions(fileNames, settings),
-    ...partialClosureJSOptions
+    // Work around https://github.com/Microsoft/TypeScript/issues/16509.
+    ...(partialClosureJSOptions as ClosureJSOptions),
   };
   // Parse and load the program without tsickle processing.
   // This is so:

test/e2e_source_map_test.ts

@@ -345,7 +345,11 @@ const DEFAULT_COMPILER_OPTIONS = {
 
 function compile(sources: Map<string, string>, partialOptions = {} as Partial<CompilerOptions>):
     {compiledJS: string, dts: string | undefined, sourceMap: SourceMapConsumer} {
-  const options: CompilerOptions = {...DEFAULT_COMPILER_OPTIONS, ...partialOptions};
+  const options: CompilerOptions = {
+    ...DEFAULT_COMPILER_OPTIONS,
+    // Work around https://github.com/Microsoft/TypeScript/issues/16509
+    ...(partialOptions as CompilerOptions),
+  };
   const resolvedSources = new Map<string, string>();
   for (const fileName of toArray(sources.keys())) {
     resolvedSources.set(ts.sys.resolvePath(fileName), sources.get(fileName)!);