retain binding sourcemaps to the extent possible

src/utils/flattenReference.js

@@ -1,5 +1,7 @@
 export default function flatten ( node ) {
 	const parts = [];
+	const propEnd = node.end;
+
 	while ( node.type === 'MemberExpression' ) {
 		if ( node.computed ) return null;
 		parts.unshift( node.property.name );
@@ -7,10 +9,11 @@ export default function flatten ( node ) {
 		node = node.object;
 	}
 
+	const propStart = node.end;
 	const name = node.type === 'Identifier' ? node.name : node.type === 'ThisExpression' ? 'this' : null;
 
 	if ( !name ) return null;
 
 	parts.unshift( name );
-	return { name, parts, keypath: parts.join( '.' ) };
+	return { name, parts, keypath: `${name}[✂${propStart}-${propEnd}✂]` };
 }

test/sourcemaps/samples/binding/test.js

@@ -1,10 +1,10 @@
 export function test ({ assert, smc, locateInSource, locateInGenerated }) {
-	const expected = locateInSource( 'foo.bar.baz' );
+	const expected = locateInSource( 'bar.baz' );
 
 	let loc;
 	let actual;
 
-	loc = locateInGenerated( 'foo.bar.baz' );
+	loc = locateInGenerated( 'bar.baz' );
 
 	actual = smc.originalPositionFor({
 		line: loc.line + 1,
@@ -18,7 +18,7 @@ export function test ({ assert, smc, locateInSource, locateInGenerated }) {
 		column: expected.column
 	});
 
-	loc = locateInGenerated( 'foo.bar.baz', loc.character + 1 );
+	loc = locateInGenerated( 'bar.baz', loc.character + 1 );
 
 	actual = smc.originalPositionFor({
 		line: loc.line + 1,