[dart2wasm] Ensure functions.getFunction().type and functions.getFunctionType() agree

Right now the two APIs disagree when the given function is a wasm
import/export: We eagerly generate wasm import/export functions and
construct their function's type in a special way - so any
`functions.getFunction()` will return this already built function
object. Though `functions.getFunctionType()` doesn't recognize
imports/exports and generates different type.

=> Make the `getFunctionType()` also recognize imports/exports.

Change-Id: I2afbf38eeda87019e589db8ee6b9f2c7697eede9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/378707
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

pkg/dart2wasm/lib/functions.dart

@@ -138,6 +138,15 @@ class FunctionCollector {
   }
 
   w.FunctionType getFunctionType(Reference target) {
+    // We first try to get the function type by seeing if we already
+    // compiled the [target] function.
+    //
+    // We do that because [target] may refer to a imported/exported function
+    // which get their function type translated differently (it would be
+    // incorrect to use [_getFunctionType]).
+    final existingFunction = getExistingFunction(target);
+    if (existingFunction != null) return existingFunction.type;
+
     return _getFunctionType(target);
   }