include location info in stringified warnings

src/Bundle.js

@@ -586,7 +586,14 @@ export default class Bundle {
 	}
 
 	warn ( warning ) {
-		warning.toString = () => warning.message || warning;
+		warning.toString = () => {
+			if ( warning.loc ) {
+				return `${warning.loc.file} (${warning.loc.line}:${warning.loc.column}) ${warning.message}`;
+			}
+
+			return warning.message;
+		};
+
 		this.onwarn( warning );
 	}
 }

src/utils/defaults.js

@@ -45,9 +45,10 @@ export function resolveId ( importee, importer ) {
 export function makeOnwarn () {
 	const warned = blank();
 
-	return msg => {
-		if ( msg in warned ) return;
-		console.error( msg ); //eslint-disable-line no-console
-		warned[ msg ] = true;
+	return warning => {
+		const str = warning.toString();
+		if ( str in warned ) return;
+		console.error( str ); //eslint-disable-line no-console
+		warned[ str ] = true;
 	};
 }