test: Match gccgo error messages.

explicit.go:36:4: error: incompatible types in assignment (need explicit conversion)
explicit.go:41:4: error: incompatible types in assignment (type has no methods)
explicit.go:42:4: error: incompatible types in assignment (need explicit conversion)
explicit.go:45:5: error: incompatible types in assignment (need explicit conversion; missing method ‘N’)
explicit.go:48:9: error: invalid type conversion (need explicit conversion; missing method ‘N’)
explicit.go:51:4: error: incompatible types in assignment
explicit.go:51:7: error: invalid type conversion (need explicit conversion)
explicit.go:57:10: error: impossible type assertion: type does not implement interface (type has no methods)
explicit.go:62:10: error: impossible type assertion: type does not implement interface (incompatible type for method ‘M’ (different number of parameters))
explicit.go:67:5: error: incompatible type in initialization (type has no methods)
explicit.go:68:5: error: incompatible type in initialization (incompatible type for method ‘M’ (different number of parameters))
explicit.go:70:11: error: invalid type conversion (type has no methods)
explicit.go:71:11: error: invalid type conversion (incompatible type for method ‘M’ (different number of parameters))

R=rsc
CC=golang-dev
https://golang.org/cl/2139044

test/interface/explicit.go

@@ -42,10 +42,10 @@ func main() {
 	t = i // ERROR "incompatible|need type assertion"
 
 	i = i2 // ok
-	i2 = i // ERROR "missing N method"
+	i2 = i // ERROR "incompatible|missing N method"
 
 	i = I(i2)  // ok
-	i2 = I2(i) // ERROR "missing N method"
+	i2 = I2(i) // ERROR "invalid|missing N method"
 
 	e = E(t) // ok
 	t = T(e) // ERROR "need explicit|need type assertion|incompatible"
@@ -64,8 +64,8 @@ var _ = m.(Int)	// ERROR "impossible type assertion"
 var ii int
 var jj Int
 
-var m1 M = ii	// ERROR "missing"
-var m2 M = jj	// ERROR "wrong type for M method"
+var m1 M = ii	// ERROR "incompatible|missing"
+var m2 M = jj	// ERROR "incompatible|wrong type for M method"
 
-var m3 = M(ii)	// ERROR "missing"
-var m4 = M(jj)	// ERROR "wrong type for M method"
+var m3 = M(ii)	// ERROR "invalid|missing"
+var m4 = M(jj)	// ERROR "invalid|wrong type for M method"