Add better Transition debugging information.

Expose a few nice debugging tools when working with a transition:

```ts
interface Transition {
  /**
    In non-production builds, this function will return the stack that this Transition was
    created within. In production builds, this function will not be present.

    @method debugCreationStack
    @return string
  */
  debugCreationStack?: () => string | undefined;

  /**
    In non-production builds, this function will return the stack that this Transition was
    aborted within (or `undefined` if the Transition has not been aborted yet). In production
    builds, this function will not be present.

    @method debugAbortStack
    @return string
  */
  debugAbortStack?: () => string | undefined;

  /**
    In non-production builds, this property references the Transition that _this_ Transition
    was derived from or `undefined` if this transition did not derive from another. In
    production builds, this property will not be present.

    @Property debugPreviousTransition
    @type {Transition | undefined}
  */
  debugPreviousTransition: Maybe<Transition<T>>;
}
```

---

Note: I've confirmed that Ember's build pipeline properly replaces the
`if (DEBUG) {` bits with `true` and `false` as appropriate:

  ```js
  if (true
      /* DEBUG */
  ) {
    var _error = new Error(`Transition creation stack`);

    this.debugCreationStack = () => _error.stack;

    this.debugPreviousTransition = previousTransition;
  }
```

lib/router/transition.ts

@@ -6,6 +6,7 @@ import TransitionAborted, { ITransitionAbortedError } from './transition-aborted
 import { OpaqueIntent } from './transition-intent';
 import TransitionState, { TransitionError } from './transition-state';
 import { log, promiseLabel } from './utils';
+import { DEBUG } from '@glimmer/env';
 
 export type OnFulfilled<T, TResult1> =
   | ((value: T) => TResult1 | PromiseLike<TResult1>)
@@ -65,6 +66,35 @@ export default class Transition<T extends Route> implements Partial<Promise<T>>
   isCausedByAbortingReplaceTransition = false;
   _visibleQueryParams: Dict<unknown> = {};
 
+  /**
+    In non-production builds, this function will return the stack that this Transition was
+    created within. In production builds, this function will not be present.
+
+    @method debugCreationStack
+    @return string
+  */
+  declare debugCreationStack?: () => string | undefined;
+
+  /**
+    In non-production builds, this function will return the stack that this Transition was
+    aborted within (or `undefined` if the Transition has not been aborted yet). In production
+    builds, this function will not be present.
+
+    @method debugAbortStack
+    @return string
+  */
+  declare debugAbortStack?: () => string | undefined;
+
+  /**
+    In non-production builds, this property references the Transition that _this_ Transition
+    was derived from or `undefined` if this transition did not derive from another. In
+    production builds, this property will not be present.
+
+    @property debugPreviousTransition
+    @type {Transition | undefined}
+  */
+  declare debugPreviousTransition: Maybe<Transition<T>>;
+
   constructor(
     router: Router<T>,
     intent: Maybe<OpaqueIntent>,
@@ -86,6 +116,17 @@ export default class Transition<T extends Route> implements Partial<Promise<T>>
     this.pivotHandler = undefined;
     this.sequence = -1;
 
+    if (DEBUG) {
+      let error = new Error(`Transition creation stack`);
+
+      this.debugCreationStack = () => error.stack;
+
+      // not aborted yet, will be replaced when `this.isAborted` is set
+      this.debugAbortStack = () => undefined;
+
+      this.debugPreviousTransition = previousTransition;
+    }
+
     if (error) {
       this.promise = Promise.reject(error);
       this.error = error;
@@ -250,9 +291,17 @@ export default class Transition<T extends Route> implements Partial<Promise<T>>
   rollback() {
     if (!this.isAborted) {
       log(this.router, this.sequence, this.targetName + ': transition was aborted');
+
+      if (DEBUG) {
+        let error = new Error(`Transition aborted stack`);
+
+        this.debugAbortStack = () => error.stack;
+      }
+
       if (this.intent !== undefined && this.intent !== null) {
         this.intent.preTransitionState = this.router.state;
       }
+
       this.isAborted = true;
       this.isActive = false;
       this.router.activeTransition = undefined;

package.json

@@ -31,7 +31,9 @@
     "start": "ember server",
     "test": "ember test"
   },
-  "dependencies": {},
+  "dependencies": {
+    "@glimmer/env": "^0.1.7"
+  },
   "devDependencies": {
     "@babel/plugin-transform-modules-amd": "^7.10.5",
     "@babel/plugin-transform-modules-commonjs": "^7.10.4",

yarn.lock

@@ -216,6 +216,11 @@
     exec-sh "^0.3.2"
     minimist "^1.2.0"
 
+"@glimmer/env@^0.1.7":
+  version "0.1.7"
+  resolved "https://registry.yarnpkg.com/@glimmer/env/-/env-0.1.7.tgz#fd2d2b55a9029c6b37a6c935e8c8871ae70dfa07"
+  integrity sha1-/S0rVakCnGs3psk16MiHGucN+gc=
+
 "@iarna/toml@2.2.5":
   version "2.2.5"
   resolved "https://registry.yarnpkg.com/@iarna/toml/-/toml-2.2.5.tgz#b32366c89b43c6f8cefbdefac778b9c828e3ba8c"