stateMock.js

angular.module('stateMock',[]);
angular.module('stateMock').service("$state", function($q){
    this.expectedTransitions = [];
    this.transitionTo = function(stateName, params){
        if(this.expectedTransitions.length > 0){
            var expectedState = this.expectedTransitions.shift();
            if(expectedState.stateName !== stateName){
                throw Error('Expected transition to state: ' + expectedState.stateName + ' but transitioned to ' + stateName );
            }
            if(expectedState.params && !angular.equals(expectedState.params, params)){
                throw Error('Expected params to be ' + JSON.stringify(expectedState.params) + ' but received ' + JSON.stringify(params));
            }
        }else{
            throw Error("No more transitions were expected! Tried to transition to "+ stateName );
        }
        console.log("Mock transition to: " + stateName);
        return $q.when();
    };
    this.go = this.transitionTo;
    this.expectTransitionTo = function(stateName, params){
        this.expectedTransitions.push({stateName: stateName, params: params});
    };

    this.ensureAllTransitionsHappened = function(){
        if(this.expectedTransitions.length > 0){
            throw Error("Not all transitions happened!");
        }
    };
});

Good stuff. I'd suggest making a small change on line 10:

if (expectedState.params && !angular.equals(expectedState.params, params)) {
  throw Error('Expected params to be ' + JSON.stringify(expectedState.params) + ' but received ' + JSON.stringify(params));
}

To ensure there's an expectation of params before making the assertion.

@moosehawk You're right. Thanks!