Covariant properties for ModuleTransportLike

Summary:
Make all properties of `ModuleTraansportLike` covariant to enforce read-only behavior at all sites using it.
The type only exists for compatibility reasons between old and new output functionality, and covariant properties give us stronger guarantees.

Reviewed By: jeanlauliac

Differential Revision: D5111667

fbshipit-source-id: 674658b07eb3a229cbc4344cb636e4a9ea4126d1

local-cli/bundle/types.flow.js

@@ -12,7 +12,7 @@
 
 import type Bundle from '../../packager/src/Bundler/Bundle';
 import type {Unbundle} from '../../packager/src/Bundler/Bundle';
-import type ModuleTransport from '../../packager/src/lib/ModuleTransport';
+import type ModuleTransport, {SourceMapOrMappings} from '../../packager/src/lib/ModuleTransport';
 import type {FBSourceMap, SourceMap} from '../../packager/src/lib/SourceMap';
 
 export type {Bundle, FBSourceMap, ModuleTransport, SourceMap, Unbundle};
@@ -24,11 +24,11 @@ export type ModuleGroups = {|
 |};
 
 export type ModuleTransportLike = {
-  code: string,
-  id: number,
-  map?: $PropertyType<ModuleTransport, 'map'>,
+  +code: string,
+  +id: number,
+  +map: ?SourceMapOrMappings,
   +name?: string,
-  sourcePath: string,
+  +sourcePath: string,
 };
 
 export type OutputOptions = {