Simplify nodesByPos fixup

Layout.js

@@ -97,15 +97,10 @@ class Layout {
           continue;
         }
         obj.savePos();
-        let nodesByPos = Array.from(this.nodesByPos_);
         obj.moveBy(offset);
         this.setTension(objects);
         let testTension = this.getTotalTension(objects);
         obj.restorePos();
-        this.nodesByPos_.clear();
-        for (let [k, v] of nodesByPos) {
-          this.nodesByPos_.set(k, v);
-        }
         if (testTension < newTension) {
           newOffset = offset;
           newTension = testTension;

LayoutGroup.js

@@ -37,6 +37,9 @@ class LayoutGroup {
     for (let node of this.nodes) {
       node.restorePos();
     }
+    for (let node of this.nodes) {
+      this.nodesByPos_.set(node.pos, node);
+    }
   }
 
   moveBy(offset) {