[analyzer] Speedup some completion queries

When getting completions, some of the "contributers" basically do a
walk of the AST and almost add everything they see, and the list
is then later pruned based on the user query.

This CL introduces a "first pruning" (a copy of the first part of what
is done in the subsequent pruning) so only the things has could match
the user query is added.

Benchmarks for this:

Notes:
 - All of these numbers are with the timeout increased a lot (to a
   point where there effectivley isn't any timeout).
 - I have an external project too, but only in Android Studio as it's
   a Flutter project and my VSCode then uses the dart included in
   flutter. I don't know how to make it not do that.

Summary:

Android Studio, bin/server.dart --- `print(CallHierarchyK^);`
Difference at 95.0% confidence
        -46.45 +/- 10.0349
        -45.7861% +/- 9.8915%
        (Student's t, pooled s = 15.6785)

Android Studio, bin/server.dart --- `starter.^;`
No difference proven at 95.0% confidence

Android Studio, Other project --- `print(Cryptm^);`
Difference at 95.0% confidence
        -90.3 +/- 10.9506
        -58.0894% +/- 7.04448%
        (Student's t, pooled s = 17.1092)

VSCode, bin/server.dart --- `print(CallHierarchyK^);`
Difference at 95.0% confidence
        -61.45 +/- 5.41501
        -51.4226% +/- 4.53139%
        (Student's t, pooled s = 8.46036)

VSCode, bin/server.dart --- `starter.^;`
Difference at 95.0% confidence
        6.4 +/- 5.08317
        15.4217% +/- 12.2486%
        (Student's t, pooled s = 7.94189)

So for several queries it's ~50% faster. For a single one it slightly slower.

Raw numbers:

BEFORE:

Android Studio, bin/server.dart --- `print(CallHierarchyK^);`
90 ms
88 ms
89 ms
85 ms
139 ms
92 ms
86 ms
101 ms
87 ms
84 ms
112 ms
110 ms
128 ms
99 ms
99 ms
104 ms
98 ms
88 ms
106 ms
144 ms

Android Studio, bin/server.dart --- `starter.^;`
38 ms
38 ms
34 ms
30 ms
35 ms
34 ms
29 ms
31 ms
35 ms
60 ms
53 ms
37 ms
54 ms
31 ms
51 ms
33 ms
39 ms
68 ms
75 ms
91 ms

Android Studio, Other project --- `print(Cryptm^);`
150 ms
146 ms
142 ms
138 ms
150 ms
213 ms
147 ms
143 ms
149 ms
147 ms
153 ms
147 ms
136 ms
151 ms
151 ms
186 ms
160 ms
153 ms
150 ms
197 ms

VSCode, bin/server.dart --- `print(CallHierarchyK^);`
122 ms
113 ms
115 ms
118 ms
124 ms
116 ms
116 ms
114 ms
118 ms
125 ms
130 ms
117 ms
104 ms
119 ms
121 ms
120 ms
127 ms
123 ms
117 ms
131 ms

VSCode, bin/server.dart --- `starter.^;`
38 ms
45 ms
35 ms
37 ms
37 ms
59 ms
39 ms
38 ms
37 ms
44 ms
41 ms
41 ms
43 ms
41 ms
39 ms
40 ms
42 ms
43 ms
49 ms
42 ms


WITH CL:

Android Studio, bin/server.dart --- `print(CallHierarchyK^);`
65 ms
59 ms
41 ms
41 ms
70 ms
38 ms
56 ms
66 ms
64 ms
44 ms
47 ms
56 ms
46 ms
38 ms
46 ms
64 ms
57 ms
44 ms
69 ms
89 ms

Android Studio, bin/server.dart --- `starter.^;`
36 ms
36 ms
38 ms
35 ms
37 ms
43 ms
32 ms
30 ms
34 ms
39 ms
32 ms
40 ms
35 ms
34 ms
35 ms
39 ms
46 ms
54 ms
56 ms
78 ms


Android Studio, Other project --- `print(Cryptm^);`
59 ms
63 ms
61 ms
80 ms
98 ms
56 ms
60 ms
61 ms
57 ms
61 ms
57 ms
61 ms
59 ms
59 ms
57 ms
64 ms
60 ms
65 ms
58 ms
107 ms


VSCode, bin/server.dart --- `print(CallHierarchyK^);`
55 ms
54 ms
50 ms
47 ms
59 ms
55 ms
53 ms
53 ms
67 ms
67 ms
54 ms
54 ms
50 ms
62 ms
54 ms
54 ms
56 ms
59 ms
63 ms
95 ms


VSCode, bin/server.dart --- `starter.^;`
57 ms
41 ms
57 ms
40 ms
45 ms
47 ms
40 ms
44 ms
43 ms
43 ms
41 ms
40 ms
42 ms
49 ms
40 ms
46 ms
57 ms
49 ms
81 ms
56 ms

https://github.com/flutter/flutter-intellij/issues/6470

Change-Id: I1e34ce04774cafcb47f4e8590b990400a7b0f0ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279389
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>