Fix nightly build after eta-expansion changes

[smarter]

The nightly build runs the sbt incremental compilation tests (that can be
run by hand using sbt "sbt-dotty/scripted source-dependencies/*" but
take a while to complete). The implicit-params test started failing
after #2701 because B.scala contains:
val y = x(3)
which was supposed to fail when the implicit is not present but now
succeeds with y getting the type E => String. We restore the failure
by adding a type annotation to y

[smarter]

This shows that we really need a warning for pure statements, like scalac:

class IOCapability

object Test {
  // Forgot to mark `ev` implicit!
  def doSideEffects(x: Int)(ev: IOCapability) = {
    println("x: " + x)
  }

  implicit val cap: IOCapability = new IOCapability

  // This will compile but won't actually produce any side-effect
  doSideEffects(1)
}

In fact, I wonder if we could make automatic eta-expansion fail when the result is a pure statement

[odersky]

Good point about the purity warning. LGTM for this PR.