Use ReportableEvent for bundle save logs

Summary:
Removes uses of `console.log` in favour of reportable events for building bundles programmatically or through the CLI.

Fixes `metro build --json` printing non-JSON output, etc.

Changelog:
```
**[Fix]** Don't use console.log directly in bundle builds
```

Reviewed By: rubennorte

Differential Revision: D65949862

fbshipit-source-id: 77fe698c15dc6ce499a1fa0e1ebc0fb39ba7814e

packages/metro/src/index.flow.js

@@ -98,7 +98,7 @@ export type RunBuildOptions = {
   onComplete?: () => void,
   onProgress?: (transformedFileCount: number, totalFileCount: number) => void,
   minify?: boolean,
-  output?: {
+  output?: $ReadOnly<{
     build: (
       MetroServer,
       RequestOptions,
@@ -114,10 +114,10 @@ export type RunBuildOptions = {
         ...
       },
       OutputOptions,
-      (...args: Array<string>) => void,
+      (logMessage: string) => void,
     ) => Promise<mixed>,
     ...
-  },
+  }>,
   platform?: string,
   sourceMap?: boolean,
   sourceMapUrl?: string,
@@ -377,7 +377,6 @@ exports.runBuild = async (
     onComplete,
     onProgress,
     minify = true,
-    // $FlowFixMe[incompatible-variance] frozen objects are readonly
     output = outputBundle,
     out,
     platform = 'web',
@@ -429,8 +428,12 @@ exports.runBuild = async (
         platform,
       };
 
-      // eslint-disable-next-line no-console
-      await output.save(metroBundle, outputOptions, console.log);
+      await output.save(metroBundle, outputOptions, message =>
+        config.reporter.update({
+          type: 'bundle_save_log',
+          message,
+        }),
+      );
     }
 
     return metroBundle;

packages/metro/src/lib/TerminalReporter.js

@@ -237,6 +237,9 @@ class TerminalReporter {
       case 'bundle_build_done':
         this._logBundleBuildDone(event.buildID);
         break;
+      case 'bundle_save_log':
+        this.terminal.log('LOG:' + event.message);
+        break;
       case 'bundle_build_failed':
         this._logBundleBuildFailed(event.buildID);
         break;

packages/metro/src/lib/reporting.js

@@ -61,6 +61,11 @@ export type ReportableEvent =
       type: 'bundle_build_failed',
       ...
     }
+  | {
+      type: 'bundle_save_log',
+      message: string,
+      ...
+    }
   | {
       buildID: string,
       bundleDetails: BundleDetails,

packages/metro/src/shared/output/bundle.flow.js

@@ -49,7 +49,7 @@ async function saveBundleAndMap(
     ...
   },
   options: OutputOptions,
-  log: (...args: Array<string>) => void,
+  log: string => void,
 ): Promise<mixed> {
   const {
     bundleOutput,