Some users during a hack night were trying to set properties this way. Since {{foo}} would never be a valid expression, the compiler should raise a helpful exception when this pattern is found. Since this pattern is predictable, the exception should give specific feedback.

My suggestion: {{foo}} is not a valid expression in [prop]="{{foo}}". Try [prop]="foo"

CC: @mhevery

Related: #3662 (should be done together)